Multi-piece golf ball

ABSTRACT

A multiple-piece golf ball includes a solid core having a specific gravity from about 1.2 to about 14; and a cover disposed on the outside of the solid core, wherein a spin decay rate of the golf ball during flight is from about 3.1% to about 20% of an initial spin rate of the golf ball from about 2800 to about 3500 rpm; and wherein a moment of inertia of the golf ball with respect to an x-axis Ix, a moment of inertia of the golf ball with respect to a y-axis Iy, and a moment of inertia of the golf ball with respect to a z-axis Iz are substantially the same, the x-axis, y-axis, and z-axis intersecting orthogonally at the center of the golf ball.

BACKGROUND OF THE INVENTION

The present invention relates to a multiple-piece golf ball.

While there are many factors that affect the distance of a golf ball, three factors, the initial velocity of the ball, the launch angle, and the spin rate are extremely important. Of these, the ball spin is an essential element in lifting the golf ball and extending its distance. However, if the spin rate is excessive, the ball can fly upward or the ball line can be short after falling, so that the distance of the ball is not extended. The spin rate is greatest at the time the ball is struck, and gradually decreases, that is, attenuates, as the ball flies.

In the Japanese Patent Application Publication No. 2007-216024, there is a description of a golf ball provided with a liquid core, and a mantle layer made from prescribed material that surrounds the liquid core, for the purpose of achieving a proper spin rate and good striking feeling. This publication describes a golf ball that has a preferable spin decay rate of at least 4% during flight with respect to the initial spin rate at the time of striking. In this publication, however, there is only language regarding a liquid-core golf ball, and no language regarding a solid-core golf ball.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a multiple-piece golf ball with a solid core, having a high spin decay rate and capable of extending the distance having a relatively large initial spin rate from about 2800 to about 3500 rpm when struck by a driver.

In order to achieve the above-noted object, the present invention is a multiple-piece golf ball including a solid core having a specific gravity from about 1.2 to about 14; and a cover disposed on the outside of the solid core, wherein a spin decay rate of the golf ball during flight is from about 3.1% to about 20% of an initial spin rate of the golf ball from about 2800 to about 3500 rpm; and wherein a moment of inertia of the golf ball with respect to an x-axis Ix, a moment of inertia of the golf ball with respect to a y-axis Iy, and a moment of inertia of the golf ball with respect to a z-axis Iz are substantially the same, the x-axis, y-axis, and z-axis intersecting orthogonally at the center of the golf ball.

The solid core may include at least one specific gravity-enhancing member. The specific gravity-enhancing member may be one and may be plural specific gravity-enhancing members. The specific gravity-enhancing member may have a transmission loss of at least about 10 dB in the region from 1000 to 4000 Hz.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing one embodiment of a multiple-piece golf ball according to the present invention.

FIG. 2 is a cross-sectional view showing another embodiment of a multiple-piece golf ball according to the present invention.

FIG. 3 is a cross-sectional view showing yet another embodiment of a multiple-piece golf ball according to the present invention.

FIG. 4 is a cross-sectional view showing still yet another embodiment of a multiple-piece golf ball according to the present invention.

FIG. 5 is a transparent view showing a further embodiment of multiple-piece golf ball according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Although embodiments of a multiple-piece golf ball according to the present invention are described below, with reference made to the accompanying drawings, the present invention is not limited to these embodiments. The attached drawings were made giving priority to understanding of the present invention, and they are not drawn to scale.

As shown in FIG. 1, a multiple-piece golf ball 1 a has a solid core 10, and a cover 30 located outside the solid core. Although an intermediate layer 20 may be provided between the solid core and the cover, as shown in FIG. 1, the present invention is not limited to this construction, and the cover 30 may directly connect to the solid score 10 without providing an intermediate layer.

The solid core 10 is not limited to a single layer configuration, as shown in the FIG. 1, but may include plural layers. For example, as shown in FIG. 2, the solid core 10 of the golf ball 1 b may be formed by two layers having the center core 12 located at the centermost of the golf ball 1 b and the core layer 14 that covers the center core 12.

On hitting the golf ball 1, when the initial spin rate is relatively large within the range from about 2800 to about 3500 rpm, the present invention provides the golf ball 1 having a configuration in which the spin decay rate while the golf ball 1 is during flight, is very high, within the range from about 3.1% to about 20%. When the initial spin rate is large as described above, if the spin decay rate is less than about 3.1%, the golf ball will climb excessively to attain too high a trajectory, so that the distance is insufficient. The golf ball often curves to the side as it drops. It is preferable that the spin decay rate be at least about 3.2%, and more preferably at least about 3.3%. On the other hand, if the spin decay rate is greater than at least about 20%, the trajectory will become unstable. The spin decay rate is preferably not greater than about 15%, and more preferably not greater than about 7%.

In order to attain such an spin decay rate, even if the solid core 10 of the golf ball 1 is at least configured by plural layers, it is required that the overall solid core have a specific gravity within the range from about 1.2 to about 14. The solid core, the intermediate layer, and the cover of the golf ball generally have a specific gravity of less than 1.2. The present invention has high specific gravity of the solid core 10 that is located at the center of the golf ball 1, thereby attaining the above-described spin decay rates.

In order to attain the spin decay rates, the specific gravity of the solid core 10 is preferably at least about 1.3, and more preferably at least about 1.8. On the other hand, the specific gravity of the solid core 10 is preferably not greater than about 14, more preferably not greater than about 13.5, still more preferably not greater than about 12.5, and most preferably not greater than about 12.

The golf ball according to the present invention has a configuration in which the moment of inertia is uniform, that is, if three axes intersecting orthogonally at the center of the golf ball are the x-axis, the y-axis, and the z-axis, and the moments of the inertia of the golf ball with respect to each axis are Ix, Iy and Iz, then Iz, Ix and Iy are substantially the same. The moment of inertia is preferably at least about 50, more preferably at least about 52, and still more preferably at least about 54. On the other hand, if the moment of inertia is too large, when the initial spin rate is large, because the ball has too high a trajectory, so that the distance is insufficient, the moment of inertia is preferably not greater than about 80, more preferably not greater than about 78, and still more preferably not greater than about 76.

As a material which forms the solid core 10 having such a specific gravity, for example, a rubber composition that includes the base rubber that is to be the main component and a filler may be used. A thermoset elastomer may be generally used as the base rubber, and although, for example, a polybutadiene rubber (BR), a styrene-butadiene rubber (SBR), a natural rubber (NR), a polyisoprene rubber (IR), a polyurethane rubber (PU), a butyl rubber (IIR), a vinyl polybutadiene rubber (VBR), an ethylene propylene rubber (EPDM), a nitryl rubber (NBR), and a silicone rubber may be used, there is no limitation to these components. The polybutadiene rubber (BR) preferably may have a 1,4-cis-bond of at least about 60%, more preferably of at least about 80%, still more preferably of at least about 90%, and most preferably at least about 95%.

The Moony viscosity (ML₁₊₄ (100° C.)) of the polybutadiene is desirably at least about 30, preferably at least about 35, more preferably at least about 40, still more preferably at least about 50, and most preferably at least about 52. The upper limit is desirably not greater than about 100, preferably not greater than about 80, more preferably not greater than about 70, and still more preferably not greater than about 60.

The Moony viscosities in the present invention indicate the industrial viscosity index (Japan Industrial Standard JIS K 6300) that is measured by a Moony viscosity gage, which is a type of a rotary plastometer, and ML₁₋₄ (100° C.) is used as an unit symbol. Symbol M indicates the Moony viscosity, L indicates the large rotor (L type), 1+4 indicates the pre-heating time of one minute and the rotor rotating time of four minutes, and it is indicates that the measurement is performed under the condition of 100° C.

A molecular weight distribution of the polybutadiene Mw/Mn (Mw: A weight average molecular weight, Mn: A number-average molecular weight) is desirably at least about 2.0, preferably at least about 2.2, more preferably at least about 2.4, and still further more preferably at least about 2.6. The upper limit is desirably not greater than about 6.0, preferably not greater than about 5.0, more preferably not greater than about 4.0, and still more preferably not greater than about 3.4. If Mw/Mn is too small, poor workability may result, and if it too large, poor repulsion may result.

Although the polybutadiene may be synthesized with a Ni or Co catalyst or with a rare-earth element catalyst, one synthesized with a rare-earth element catalyst is particularly preferable, and a widely known rare-earth element catalyst can be used. Exemplary catalysts include those made up of a combination of a lanthanide series rare-earth element compound, an organoaluminum compound, an alumoxane, a halogen-bearing compound and, if necessary, a Lewis base.

In the present invention, the use of a neodymium catalyst in which a neodymium compound serves as the lanthanide series rare-earth element compound is particularly advantageous because it enables a polybutadiene rubber having a high 1,4-cis-bond content and a low 1,2-vinyl bond content to be obtained with excellent polymerization activity. Specific examples of these rare-earth element catalysts include those noted in Japanese Patent Application Publication No. 11-35633, which is incorporated by reference.

In the case of the butadiene polymerization in the presence of a rare-earth element catalyst, a solvent may be used or bulk polymerization or vapor phase polymerization may be preferable, without using the solvent, and the polymerization temperature can be generally from about −30° C. to about 150° C., and preferably from about 10° C. to about 100° C.

The above-noted butadiene may be obtained by following polymerization by the above-noted rare-earth element catalyst by reacting a terminal modifier with the activation terminal of the polymer. The specific embodiment of the terminal modifier and the method of activation include, for example, those noted and methods in the Japanese Patent Application Publications 11-35633, 7-268132, and 2002-293996, which are incorporated by reference.

The butadiene is preferably to be blended at at least about 60 wt % into the base rubber, more preferably at at least about 70 wt %, still more preferably at at least about 80 wt %, and most preferably at at least 90 wt %. The upper limit of the mixture ratio of the butadiene is preferably about 100 wt %, more preferably about 98 wt %, and still more preferably about 95 wt %. By mixing the polybutadiene within this range, a golf ball having good resilience can be obtained.

Rubbers other than the above-described polybutadiene may be blended in combination with the polybutadiene, insofar as the object of the present invention is not lost. Particularly preferable rubbers for the combination include a styrene-butadiene rubber, a natural rubber, a polyisoprene rubber, and an ethylene-propylene-diene rubber. These may be used alone or in combination of two or more thereof.

A metal or metallic compound with a specific gravity of at least 2 is desirable as a filler. This because if the specific gravity of the filler is less than about 2, the degree of freedom in designing the specific gravity of the solid core 10 at least 1.2 is low. Although examples of such a metal or a metallic compound include silver, gold, cobalt, chromium, copper, iron, germanium, manganese, molybdenum, nickel, lead, platinum, tin, titanium, tungsten, zinc, zirconium, barium sulfate, zinc oxide, and manganese oxide, there is no particular limitation thereto. The filler is preferably formed as a powder.

The specific gravity of the filler is preferably at least about 2.5, and more preferably at least about 4.0. If the specific gravity of the filler is too high, only a small non-uniform dispersion of the filler can be found within the solid core 10, and an unbalanced weight of the solid core 10 occurs, and because it is difficult, from the standpoint of the manufacturing process, to achieve a uniform moment of inertia, the specific gravity of the filler is preferably not greater than about 22, and more preferably not greater than about 20.

The filler is to be blended so that the specific gravity of the solid core 10 is within the above-noted range. Although there is no particular limitation with respect to the filler content, for example, per 100 parts by weight of the base rubber, it is preferably at least about 10 parts by weight, more preferably at least about 15 part by weight, and still more preferably at least about 25 parts by weight. The upper limit of the filler content is preferably about 1000 parts by weight, more preferably about 980 parts by weight, and still more preferably about 960 parts by weight.

The rubber composition which the materials to form the solid core 10 may optionally include a co-crosslinking agent, an initiator, a foaming agent, an antioxidant, an organosulfur compound, and a vulcanization accelerator. Although there is no particular limitation with regard to the co-crosslinking agent, it is preferable that, for example, an α,β-unsaturated carboxylic acid or a metal salt thereof be used. As an α,β-unsaturated carboxylic acid, or the metal salt, there are, for example, an acrylic acid, methacrylate, and a zinc salt, a magnesium salt, and a calcium salt thereof. Although there is no particular limitation with regard to the co-crosslinking agent content, for example, per 100 parts by weight of the base rubber, it is preferably at least about 5 parts by weight, and more preferably at least about 10 parts by weight. The co-crosslinking agent content is preferably not greater than about 70 parts by weight, and more preferably not greater than about 50 parts by weight.

Although no particular limitation is imposed on the initiator, the use of an organic peroxide is preferable. Although no particular limitation is imposed on the initiator content, for example, per 100 parts by weight of the base rubber, it is preferably at least about 0.10 parts by weight, more preferably at least about 0.15 parts by weight, and still more preferably at least about 0.30 parts by weight, but preferably not greater than 8 parts by weight, and more preferably not greater than 6 parts by weight.

Although no particular limitation is imposed on the foaming agent, for example, an azodicarbonamide, azobisisobutyronitrile, dinitrosopentamethylenetetramine, p-toluenesulfonyl hydrazide, p,p′-oxybis(benzenesulfonylhydrazide), and sodium hydrogencarbonate may be used. Although no limitation is imposed on the foaming agent content, for example, per 100 parts by weight of the base rubber, it is preferably at least about 5 parts by weight, and more preferably at least about 10 parts by weight. The foaming agent content is preferably not greater than about 30 parts by weight, and more preferably not greater than about 25 parts by weight.

The solid core 10 shape is substantially spherical. The outside diameter of the solid core 10 is preferably not greater than about 40 mm in order to set the moment of inertia of the golf ball with a weight of 45.5 g to not greater than 80. The outer diameter of the solid core 10 is preferably not greater than about 39 mm, or more preferably not greater than about 36 mm. On the other hand, because if the outside diameter of the solid core 10 is too small, the repulsion of the golf ball is reduced, it is desirable at least about 4 mm, preferably at least about 5 mm, more preferably at least about 6 mm, still more preferably at least about 7 mm, and most preferably at least about 8 mm.

As shown in FIG. 2, in the case in which the solid core 10 is made of a multilayer structure including the center core 12 and core layer 14, the thickness of the core layer 14 is preferably at least about 0.8 mm, more preferably at least about 1.0 mm, and still more preferably at least about 1.4 mm. The upper limit of the thickness of the core layer 14 is desirably about 10 mm, preferably about 8 mm, more preferably about 6 mm, and still more preferably about 3 mm.

As shown in FIG. 3 to FIG. 5, the solid core 10 may also include one or plural specific gravity-enhancing members 40, 42, and 44. The metal composition having a higher specific gravity than the rubber composition for forming the above-described solid core is used as the specific gravity-enhancing member. This enables the specific gravity of the solid core 10 to more easily be made at least 1.2.

The specific gravity of the specific gravity-enhancing member is designed for at least a specific gravity of 1.2 for the solid core. From the standpoint of the degree of freedom, it is preferably at least about 2, more preferably at least about 2.5, and still more preferably at least about 4. On the other hand, if the specific gravity of the specific gravity-enhancing member is too high, only a small un-uniform arrangement of the specific gravity-enhancing member can be found within the solid core, an unbalanced weight of the soled core occurs, and because it is difficult in the manufacturing process to achieve a uniform moment of inertia, the specific gravity of the specific gravity-enhancing member is preferably not greater than about 22, and more preferably not greater than about 20.

The metal composition may include a metal or a metallic compound. Although exemplary metal or a metallic compounds include silver, gold, cobalt, chromium, copper, iron, germanium, manganese, molybdenum, nickel, lead, platinum, tin, titanium, tungsten, zinc, zirconium, barium sulfate, zinc oxide, and manganese oxide, there is no particular limitation imposed thereon.

The metal composition may also include binder for forming the powder of the above-noted metals or the metal compound into a prescribed shape. The use of a rubber or a thermoset elastomer as the binder is preferable, and the same materials as the above-described rubber bases can be used. Although no limitation is imposed on the powder content of the above-noted metal or the metal compound, for example, per 100 parts by weight of the base rubber, its preferably at least about 10 parts by weight, more preferably at least about 15 part by weight, and still more preferably at least about 25 parts by weight. The upper limit of the powder content of the metal or the metal compound is preferably about 1000 parts by weight, more preferably about 980 parts by weight, and still more preferably about 960 parts by weight.

Although no limitation is imposed on the shape of the specific gravity-enhancing member, for example, a spherical shape or spherical shell-shape is desirable. For example, as shown in FIG. 3, the spherical specific gravity-enhancing member 40 may be arranged at the center of the golf ball 1 c. In this example, the solid core includes this spherical specific gravity-enhancing member 40 and the spherical shell-shaped rubber layer 16 that is formed by using the above-described rubber composition therearound. Although the method of forming the spherical shell-shaped rubber layer 16 is not limited by this method, for example, the rubber composition is pre-molded in the shape of two half-cups, the spherical specific gravity-enhancing member 40 is put into these two cups, hot forming being done.

The outside diameter of the spherical specific gravity-enhancing member 40 which is disposed at the center of the solid core is desirably at least about 4 mm, preferably at least about 5 mm, more preferably at least about 6 mm, still more preferably at least about 7 mm, and most preferably at least about 8 mm. On the other hand, because too large an outer diameter of the specific gravity-enhancing member 40 makes the sagging hardness of the golf ball high or the sound occurring when the ball is struck high enough to be unpleasant, the outside diameter of the specific gravity-enhancing member 40 is desirably not greater than about 22 mm, preferably not greater than about 20 mm, more preferably not greater than about 18 mm, still more preferably not greater than about 16 mm, and most preferably not greater than about 14 mm.

The spherical shell-shaped specific gravity-enhancing member 42 may also be disposed concentrically with the center of the golf ball 1 d, as shown in FIG. 4. In this example, the solid core includes this spherical shell-shaped specific gravity-enhancing member 42 and the center core 12 and the core layer 14 inside and outside thereof, which are formed by using the above-described rubber composition. Although no limitation is imposed on this spherical shell-shaped specific gravity-enhancing member 42, it is possible, for example, to form it by subjecting the surface of the center core 12 to metal plating.

The thickness of such a spherical shell-shaped specific gravity-enhancing member 42 is desirably at least about 0.01 mm, preferably at least about 0.02 mm, more preferably at least about 0.09 mm, and most preferably at least about 0.10 mm. On the other hand, because too thick a spherical shell-shaped specific gravity-enhancing member 42 makes the sagging hardness of the golf ball high or the sound occurring when the ball is struck high enough to be unpleasant, the thickness of the spherical shell-shaped specific gravity-enhancing member 42 is desirably not greater than about 2.0 mm, preferably not greater than about 1.75 mm, more preferably not greater than about 0.75 mm, and most preferably not greater than about 0.5 mm.

Because disposing the spherical shell-shaped specific gravity-enhancing member 42 nearer the center of the golf ball 1 d, makes the smaller moment of inertia of the golf ball easier to design, the distance from the center of the ball to the spherical shell-shaped specific gravity-enhancing member 42 is desirably not greater than about 20 mm, preferably not greater than about 19 mm, and more preferably not greater than about 18 mm. Although the lower limit of the distance from the center of the ball to the spherical shell-shaped specific gravity-enhancing member 42 does not have any particular limitation, for example, it is desirably 4 mm, preferably 5 mm, and more preferably 8 mm.

No limitations of the position and number of the spherical shell-shaped specific gravity-enhancing member 42 are imposed on FIG. 4, and for example, the spherical shell-shaped specific gravity-enhancing member 42 may be disposed at the outermost position of the solid core, that is, the position directly connecting with the intermediate layer 20 or the cover 30, and plural spherical shell-shaped specific gravity-enhancing members with different size outside diameters may also be disposed by sandwiching a core layer.

In order to achieve a uniform moment of inertia of the golf ball, plural specific gravity-enhancing members 44, as shown in FIG. 5, may also be disposed at the vertices or on the surfaces of a regular polyhedron, such as a regular tetrahedron with the center of gravity at the center of the golf ball 1 e. In this manner, plural specific gravity-enhancing members 44 are disposed, thereby preventing the possibility of cracking of the specific gravity-enhancing member at the time of impact, and the possibility of hardening the golf ball, in contrast with the case of using a single member rather than the above-described plurality of specific gravity-enhancing members.

This is because when the number of specific gravity-enhancing members 44 is greater, it is easier to achieve a uniform moment of inertia of the golf ball, for example the number of four, six, eight, ten, twelve, and twenty members may be possible, and these specific gravity-enhancing members 44 may also be disposed at the vertices of a regular polyhedron, such as a regular tetrahedron, a cube, a regular octahedron, a regular decahedron, a regular dodecahedron, or a regular icosahedron. Although the upper limit of the number of specific gravity-enhancing members 44 is not particularly limited, it is preferably about 600, and more preferably about 500.

If the plurality of specific gravity-enhancing members 44 is disposed at a position near the center of the golf ball 1 e, a golf ball having low moment of inertia can be easily designed. Therefore, the distance from the center of the golf ball to the specific gravity-enhancing member 44 is preferably not greater than about 20 mm, more preferably not greater than about 19 mm, and still more preferably not greater than about 18 mm. The lower limit of the distance of the from the center of the golf ball to the specific gravity-enhancing member 44, although not limited, is preferably about 4 mm, more preferably about 5 mm, and still more preferably about 8 mm.

Although the shape of this plurality of specific gravity-enhancing members 44 is not particularly limited, it can be made spherical or sheetlike. When using the sheet type, regardless of the shape, because if the thicknesses of the plurality of specific gravity-enhancing members 44 is excessive, the sagging hardness of the golf ball high is high or the sound occurring when the ball is struck is high enough to be unpleasant, the thickness is preferably not greater than about 2.0 mm, more preferably not greater than about 1.75 mm, still more preferably not greater than about 0.75 mm, and most preferably not greater than about 0.5 mm. On the other hand, the thicknesses of the plurality of specific gravity-enhancing members 44 is preferably at least about 0.01 mm, more preferably at least about 0.02 mm, still more preferably at least about 0.09 mm, and most preferably at least about 0.10 mm.

The disposition of the plurality of specific gravity-enhancing members 44 may not be limited to the vertices or on the surfaces of the regular polyhedron as shown in FIG. 5, and if the moment of the golf ball is uniform, other arrangements may be adopted. For example, it is possible that the plurality of specific gravity-enhancing members having different specific gravities, weights, sizes and the like are disposed so as to be at different distances from the center of the golf ball in order for the inertia moment of the golf ball to be uniform.

If the transmission loss of the material is large in the high frequency range of 1000 Hz to 4000 Hz, because a sound insulation effect can be expected, the transmission loss of the specific gravity-enhancing members is preferably at least about 10 dB in the high frequency range of 1000 Hz to 4000 Hz, more preferably at least about 15 dB, and still more preferably at least about 30 dB. On the other hand, it is preferably not greater than about 60 dB, more preferably not greater than about 55 dB, and still more preferably not greater than about 40 dB.

Although the intermediate layer 20 is, as shown in FIG. 2, configured of a single layer, it is not limited to this configuration, and may be configured of at least two layers. Although the material of the intermediate layer 20 is not limited thereto, the following components (a) and/or (b) are to be the base resins. The optional component (c) may also be added to the base resin. The component (a) includes an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer and/or the metal salts thereof, the component (b) includes an olefin-unsaturated carboxylic acid random binary copolymer and/or the metal salts thereof, and the component (c) includes crystalline polyolefin blocks, and a thermoplastic block copolymer having a polyethylene/butylene random copolymer.

The foaming agent is added into the material of the intermediate layer 20, thereby providing space in the intermediate layer 20. The above-described foaming agent may be used as the foaming agent. The foaming agent content is not limited to the above, and may preferably, for example, be at least about 2 parts by weight, and more preferably at least about 3 parts by weight, per 100 parts by weight of the above-noted heated mixture. The foaming agent content is also preferably not greater than about 30 parts by weight, and more preferably not greater than about 25 parts by weight.

The weight-average molecular mass (Mw) of an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer and/or the metal salts forming the component (a) is preferably at least about 100,000, more preferably at least about 110,000, and still more preferably at least about 120,000. The upper limit is preferably about 200,000, more preferably about 190,000, and still more preferably about 170,000. The ratio of the weight-average molecular mass (Mw) and the number-average molecular mass (Mn) of the above-noted copolymer is preferably about 3.0 to about 7.0.

The component (a) is a copolymer including an olefin, for example, in which the number of carbons is at least 2 as the olefin in the component (a), but the upper limit is preferably 8, and particularly preferably 6. Specific examples include ethylene, propylene, butene, pentene, hexene, heptene, octene and the like. Ethylene is particularly preferred.

Examples of unsaturated carboxylic acids in the component (a) include acrylic acid, methacrylic acid, maleic acid, and fumaric acid. Acrylic acid and methacrylic acid are particularly preferred.

The unsaturated carboxylic acid ester in the component (a) includes, for example, a lower alkyl ester of the above-described unsaturated carboxylic acid. Specific examples include methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, methyl acrylate, ethyl acrylate, propyl acrylate and butyl acrylate. Butyl acrylate (n-butyl acrylate, i-butyl acrylate) is particularly preferred.

The random copolymer in the component (a) can be obtained by random copolymerizing the above-noted materials by a known method. The content of unsaturated carboxylic acid (acid contents) present in the random copolymer is usually at least about 2 wt %, preferably at least about 6 wt %, and more preferably at least about 8 wt %. The upper limit may be about 25 wt %, is preferably about 20 wt %, and more preferably about 15 wt %. If the acid content is low, the resilience may decrease, whereas if it is high, the workability of the material may decrease.

The metal ion of the copolymer in the component (a) can be obtained by partially neutralizing the acid groups on the random copolymers of the component (a) with metal ions.

Illustrative examples of metal ions for neutralizing the acid groups include ions such as Na, K, Li, Zn, Cu, Mg, Ca, Co, Ni, and Pb, and are preferably ions, such as, Na, Li, Zn, Mg, and Ca, and Zn ion is more preferably recommended. Although the degree of the neutralization of the random copolymer in these metal ions is not particularly limited, it is normally at least about 5 mol %, preferably at least about 10 mol %, particularly at least about 20 mol %. The upper limit is usually about 95 mol %, preferably 90 mol %, and particularly about 80 mol %. If the degree of neutralization exceeds about 95 mol %, formability may decrease. In a case in which the upper limit is about 5 mol %, because it is necessary to increase the additive amount of inorganic metal compounds to the component (e), this might be disadvantageous in view of cost. Such neutralization material can be obtained using a known method, for example, a compound such as a formate, acetate, nitrate, carbonate, bicarbonate, oxide, hydroxide or alkoxide of the above-noted metal ions being introduced with respect to the above-noted random copolymer.

Illustrative examples of the olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer which constitutes the component (a) include particularly, in terms of commercial names, “Nucrel AN 4318”, “Nucrel AN4319”, and “Nucrel AN4311” (all products of DuPont-Mitsui Polychemicals Co., Ltd.). Illustrative examples of the metal salts of the olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer include specifically, in terms of commercial names, “Himilan AM7316”, “Himilan AM7331”, “Himilan 1855” and “Himilan 1856” (all products of DuPont-Mitsui Polychemicals Co., Ltd.) and, in terms of commercial names, “Surlyn 6320” and “Surlyn 8120” (all products of DuPont US).

The weight average molecular mass (Mw) of one or more of an olefin-unsaturated carboxylic acid random binary copolymer and the metal salts thereof, which constitutes the component (b), is preferably at least about 100,000, more preferably at least about 110,000 and still more preferably at least about 120,000. The upper limit is preferably about 200,000, more preferably about 190,000, and still more preferably not more than about 170,000. The ratio of the weight-average molecular mass (Mw) and the number-average molecular mass (Mn) of the above-noted copolymer is preferably about 3.0 to about 7.0.

The ratio in which the copolymer of the component (6) occupies the overall base resin is about 0 wt % to about 20 wt %. The lower limit is preferably about 1 wt %. The upper limit is preferably about 17 wt %, more preferably about 10 wt %, more preferably about 8 wt %, and still more preferably about 5 wt %.

Illustrative examples of the olefin-unsaturated carboxylic acid random binary copolymer and the metal salts thereof, which constitute the component (b), include specifically, in terms of commercial names, “Nucrel 1560”, “Nucrel 1525”, and “Nucrel 1035” (all products of DuPont-Mitsui Polychemicals Co., Ltd.). Illustrative examples of the metal salts of the olefin-unsaturated carboxylic acid random binary copolymer include specifically, in terms of commercial names, “Himilan 1605”, “Himilan 1601”, “Himilan 1557”, “Himilan 1705” and “Himilan 1706” (all products of DuPont-Mitsui Polychemicals Co., Ltd.), and in terms of commercial names, “Surlyn 7930” and “Surlyn 7920” (all products of DuPont US).

Illustrative examples of the thermoplastic block copolymer having crystalline polyolefin blocks, or polyethylene/butylene random copolymers as the component (c) include crystalline polyethylene blocks (E) as the hard segment, and blocks which constitutes the relatively random copolymer (EB) of the ethylene and the butylene as the soft segment. A block copolymer having an E-EB type structure, or an E-EB-E type structure or the like in which the hard segment exists at one end or both ends as a molecular structure is preferably used.

The components (c), that is, the thermoplastic block copolymer having crystalline olefin blocks or polyethylene/butylene random copolymer can be obtained, for example, by hydrogenating the polybutadiene. The polybutadiene used for hydrogenating particularly has in a blocking manner a 1,4-polymerization part in which the 1,4-bond is about 95 to 100 wt %, and the polybutadiene in which the 1,4-bond content in the overall butadiene structures is about 50 wt % to 100 wt %, and more preferably about 80 wt % to 100 wt % is appropriately used as a bonding mode in the butadiene structure. That is, the polybutadiene in which the 1,4-bond content is about 50 wt % to 100 wt %, and more preferably about 80 wt % to 100 wt % and the polybutadiene having in a blocking manner about 95 to 100 wt % of 1,4-bond are preferably used.

The above-noted E-EB-E type thermoplastic block copolymer is preferably obtained by hydrogenating the polybutadiene in which the ends of the molecular chain is 1,4-bond-rich 1,4-polymer and the 1,4-bond and 1,2-bond are mixed in the intermediate part. In this case, the amount of hydrogen additive (the conversion rate to the saturated bond of the double bond in the polybutadiene) in the hydrogen additive is preferably about 60% to about 100%, and more preferably about 90% to 100%. If the amount of the hydrogen additive is too small, deterioration such as gelatinization may be caused in the blending process with ionomer resin and the like. Also, there might be a problem of an impact resistance as an intermediate layer, caused when forming the golf ball.

In the block copolymer having an E-EB type, an E-EB-E type structure or the like in which the hard segment exists at one end or both ends as a molecular structure, which is preferably used as the thermoplastic block copolymer, the amount of hard segment is preferably about 10 wt % to about 50 wt %. If the amount of the hard segment is excessive, the object of the present invention may not be achieved effectively due to the lack of flexibility, and if the amount of the hard segment is too small, a problem of formability of the blend may be caused.

The melt index of the above thermoplastic block copolymer at 230° C., and a test load of 21.2 N is preferably about 0.01 g/10 min to about 15 g/10 min, and more preferably about 0.03 g/10 min to about 10 g/10 min. In the case of exceeding the above-noted range, problems at the time of the injection molding, such as a weld, a sink, a short or the like may occur. The surface hardness of the thermoplastic block copolymer is preferably 10 to 50. If the surface hardness is too low, the durability with respect to repeated impact of the golf ball may decrease. On the other hand, if the surface hardness is too high, the resilience of the blend with the ionomer resin may decrease. The number-average molecular weight of the thermoplastic block copolymer is preferably from about 30,000 to about 800,000.

A commercially available product may be used as the thermoplastic block copolymer having the above crystalline polyolefin blocks, or a polyethylene/butylene random copolymer, and illustrative examples include Dynaron 6100P, 6200P and 6201B of JSR Corporation. In particular, Dynaron 6100P is a block polymer having crystalline olefin blocks at the ends, and may be used preferably in the present invention. These olefin series thermoplastic elastomers may be used as one type alone or may be used by blending with two or more types.

The material of the intermediate layer 20, as the component (d) with respect to 100 parts by weight of the above-noted resin components (a) to (c), can be mixed at about 5 to 100 parts by weight of a fatty acid or derivative thereof having a molecular weight from about 280 to about 1500, and, as the component (e), can be mixed with from about 0.1 parts by weight to about 10 parts by weight of the basic inorganic metal compounds, which can neutralize acid groups in the above-noted components (a), (b) and (d).

Component (d) is a fatty acid or a fatty acid derivative having a molecular weight of at least 280, but no greater than 1500. Compared with the above-noted components (a) to (c), this component has a very low molecular weight, and by contributing a prominent increase in the melt viscosity of the mixture, helps in improving the flow properties of heated mixture. The fatty acid (or derivatives thereof) of the component (d) includes a relatively high content of acid groups (or derivatives thereof), and its addition suppresses an excessive loss in resilience.

The fatty acid or fatty acid derivative of component (d) may be an unsaturated fatty acid (or derivative thereof) containing a double bond or triple bond on the alkyl group, or it may be a saturated fatty acid (or derivative thereof) in which the bonds on the alkyl group are only single bonds. It is recommended that the number of carbons in one molecule be usually at least 18, the upper limit being 80, and in particular, 40. Fewer carbons may deteriorate the heat resistance and may also make the acid group content so large as not to prevent the attainment of the desired flowability due to interactions with acid groups present in the base resin. On the other hand, if there are many carbons, the molecular weight increases, so that the flowability may decrease and make use as a material difficult.

As the fatty acid of component (d), specifically, use of stearic acid, 12-hydroxystearic acid, behenic acid, oleic acid, linoleic acid, linolenic acid, arachidic acid and lignoceric acid can be exemplified, particularly, stearic acid, arachidic acid, behenic acid and lignoceric acid is preferred.

One replaced with a proton included in the acid group of the fatty acid may be cited as the fatty acid derivative of component (d), and metallic soaps replaced with a metal ion may be cited as an example thereof. As examples of the metal ion for use in metallic soaps, ions such as Li, Ca, Mg, Zn, Mn, Al, Ni, Fe, Cu, Sn, Pb, and Co may be cited. Of these ions, Ca, Mg, and Zn are particularly preferred. The Fe ion may be bivalent or trivalent.

Specific examples of fatty acid derivatives that may be used as component (d) include a magnesium stearate, calcium stearate, zinc stearate, magnesium 12-hydroxystearate, calcium 12-hydroxystearate, zinc 12-hydroxystearate, magnesium arachidate, calcium arachidate, zinc arachidate, magnesium behenate, calcium behenate, zinc behenate, magnesium lignocerate, calcium lignocerate and zinc lignocerate. In particular, the magnesium stearate, calcium stearate, zinc stearate, magnesium arachidate, calcium arachidate, zinc arachidate, magnesium behenate, calcium behenate, zinc behenate, magnesium lignocerate, calcium lignocerate and zinc lignocerate may be preferably used.

It is recommended that component (d) be included in the amount, per 100 parts by weight of the above-noted base resin, of usually at least about 5 parts by weight, preferably at least about 8 parts by weight, more preferably at least about 20 parts by weight, and still more preferably at least 40 parts by weight, and an upper limit, usually about 100 parts by weight, preferably about 90 parts by weight, more preferably about 80 parts by weight, and still more preferably about 70 parts by weight.

In the case of using one or more of the above-noted components (a) and (d), a widely known metal soap-modified ionomer may be used (U.S. Pat. No. 5,312,857, U.S. Pat. No. 5,306,760 and PCT International Publication No. 98/46671 being incorporated by reference).

Component (e) may be added as a basic inorganic metal compound capable of neutralizing acid groups in above-noted components (a), (b) and (c). As described in the conventional example, when components (a), (b) and (d) alone, particularly, only a metal modified ionomer resin (e.g., the metal soap-modified ionomer resins alone cited in the above-noted patent publications) are used, as shown below, the metallic soap and unneutralized acid groups present on the ionomer undergo exchange reactions during mixture under heating, thereby generating a fatty acid. Because the thus-generated fatty acid has a low thermal stability and readily vaporizes during molding, it may cause molding defects, and the fatty acid thus-generated is deposited on the surface of the molded material, and this may cause a substantially lower paint film adhesion. Component (e) will be mixed in to be solved these problems.

The heated mixture used in the present invention is a blend, as described above, including compound (e) as an essential ingredient, the basic inorganic metal compound neutralizing acid groups included in the above-noted components (a), (b) and (d). By blending component (e), the acid groups in the above components (a), (b) and (d) are neutralized and, through synergistic effects from the blending of each of these components, this contributes as well to an increase the thermal stability of the heated mixture and gives it good moldability, and also enhances the resilience as a golf ball.

The component (e) is a basic inorganic metal compound capable of neutralizing acid groups within the above-noted components (a), (b) and (d), and is preferably recommended as a monoxide or hydroxide, which has a high reactivity with the ionomer resin and contains no organic acids in the reaction by-products, thus enabling the degree of neutralization of the heated mixture to be increased without a loss of thermal stability.

As the metal ions used in the basic inorganic metal compounds, ions, for example, Li, Na, K, Ca, Mg, Zn, Al, Ni, Fe, Fe, Cu, Mn, Sn, Pb, and Co can be cited, and as an inorganic metal compound there are basic inorganic fillers including these metallic ions, specifically, a magnesium oxide, magnesium hydroxide, magnesium carbonate, zinc oxide, sodium hydroxide, sodium carbonate, calcium oxide, calcium hydroxide, lithium hydroxide and lithium carbonate can be cited. A monoxide or a hydroxide is recommended as noted above. The magnesium oxide and calcium hydroxide, which have a high reactivity with the ionomer resin, may be preferably used.

The component (e) is usually about 0.1 to about 10 parts by weight, per 100 parts by weight of the above-noted base resin. The lower limit is preferably about 0.5 parts by weight, more preferably about 1 part by weight. The upper limit is preferably about 5 parts by weight, more preferably about 3 parts by weight.

The heated mixture to be used in the present invention is mixed with the above-described components (a) to (e), this improving thermal stability, moldability and resilience, and it is recommended that at least about 70 mol %, preferably at least about 80 mol %, and more preferably at least about 90 mol %, of the acid groups within every heated mixture to be used in the present invention be neutralized. Such a high degree of neutralization makes it possible to more reliably suppress the exchange reactions that cause a problem when only above-noted components (a), (b) and a fatty acid (derivative) are used as described above, thus preventing the generation of fatty acid and dramatically increasing thermal stability, enabling products of good formability and very excellent resilience compared to prior-art ionomer resins.

In this case, for the neutralization of the heated mixture of the present invention to achieve more reliably both a high degree of neutralization and flowability, it is recommended that the acid groups in the above-noted heated mixture be neutralized with transition metal ions and with at least one of alkali metal and alkaline earth metal ions. Because neutralization with transition metal ions results in a weaker ionic cohesion than with alkali (earth) metal ions, a part of the acid groups within the heated mixture is neutralized to be able to achieve a prominent improvement in flowability.

Various optional additives may be added, if necessary, in the above-noted heated mixture in the present invention, for example, fillers, pigments, dispersants, antioxidants, ultraviolet absorbers, and light stabilizers. In order to improve the feel of the ball upon impact, in addition to the above-noted essential components, various non-ionomeric thermoplastic elastomers may be blended, for example, styrene thermoplastic elastomers, ester thermoplastic elastomers, and urethane thermoplastic elastomers, and the use of styrene thermoplastic elastomers is particularly preferred.

A method of adjusting the heated mixture, for example, is mixing during heating using in internal mixer, such as a two-screw extruder, Banbury mixer, kneader, and the like, under the conditions for heating and mixing at the temperature of, for example, about 150° C. to about 250° C. The method of forming the intermediate layer using the above-noted heated mixture is not particular limited to the above, and it may be formed using, for example, injection molding or compression forming, or the like. In the case in which the injection molding method is used, after disposing the prefabricated solid core at the predetermined position of the injection mold, the method for introducing the above-noted material into the mold may be used. In the case in which the compression forming method is used, after forming one pair of half-cups using the above-noted material, the cups surround the core directly or via the intermediate layer, thereby enabling pressurizing and heating thereof within the mold. When applying pressure and heat for forming the ball, the conditions of the temperature at about 120° C. to about 170° C. and the time period of about 1 to 5 minutes may be used.

The specific gravity of the intermediate layer 20 is not limited, but may be, for example, preferably at least about 0.2, more preferably at least about 0.3, and still more preferably at least about 0.4. The upper limit of the intermediate layer 20 is desirably about 1.2, preferably about 1.15, more preferably about 1.1, still more preferably about 1.08, and most preferably about 1.06. The thickness of the intermediate layer 20 is not limited but may be, for example, preferably at least about 1 mm, more preferably at least about 2 mm, and still more preferably at least about 3 mm. The upper limit of the thickness of the intermediate layer 20 is preferably about 15 mm, more preferably about 13 mm, and still more preferably about 11 mm.

The intermediate layer 20 may be formed so as to have, as shown in FIG. 2, the spaces 22 and 24 that are not filled with material. Although the spaces, for example, include the space 22 in which the intermediate layer 20 forms a dent with respect to the cover 30, and the hole-shaped space 24 which passes through the intermediate layer 20, there is no particular limitation to the above shapes. The space in the intermediate layer 20 reduces the weight of the intermediate layer 20 and enables making the specific gravity of the solid core 10 high.

The ratio of volumes of the space to volumes of the overall intermediate layer 20 is preferably at least about 10%, more preferably at least about 15%, and still more preferably at least about 25%. This ratio is preferably not greater than about 90%, more preferably not greater than about 85%, still more preferably not greater than about 70%, and most preferably not greater than about 60%.

Although the method of forming the intermediate layer 20 having the space is not limited, it may include, for example, the method of inserting and molding the materials of the solid core and the intermediate layer, and the method of shaping the two half-cups in which materials of the intermediate layer have spaces or dents and linking these together over the solid core. Also, this method may be repeated, thereby forming the intermediate layer having a multiple layer structure in which layers having holes or dents is lamented. It will be understood, of course, that the above two methods may be combined.

The surface of the cover 30 forms plural dimples 32. Although the material of the cover 30 is not limited, an ionomer resin, polyurethane thermoplastic elastomers, a thermoset polyurethane, or mixture thereof can be used. To the cover 30 may be added the main components of the above-noted ionomer resin, polyurethane thermoplastic elastomers, or the thermoset polyurethane, and other thermoplastic elastomers, polyisocyanate compounds, fatty acid or fatty acid derivative, basic inorganic metal compounds, filler, or the like may also be added.

Although the specific gravity of the cover 30 is not limited, it is preferably at least about 0.91, and more preferably at least about 0.93, but preferably not greater than about 1.3, and more preferably not greater than about 1.2. Although the thickness of the cover 30 is not limited, it is preferably at least about 0.2, and more preferably at least about 0.4, but preferably not greater than about 4 mm, more preferably not greater than about 3 mm, and still more preferably not greater than about 2 mm.

The number of dimples 32 in the overall surface of the golf ball 1 is preferably at least about 200, more preferably at least about 250, still more preferably at least about 280, and most preferably at least about 300. The upper limit of the number of the dimples 32 is preferably about 450, more preferably about 430, and still more preferably about 410. The number of dimples 32 is set within this range, thereby enabling the golf ball 1 to receive a lifting force, and to increase the distance, particularly when using a driver.

Various types of dimples 32 having one or more of different sizes of diameters and depths may be formed. The number of types of dimples is preferably at least about 4 types, more preferably at least about 5 types, and still more preferably at least about 6 types. The upper limit thereof is preferably about 20 types, more preferably about 15, and still more preferably about 12. By making the number of types of dimples within this range, it is possible to facilitate an increase in the dimple-occupied surface ratio, and to improve the distance.

The shape of the dimple 32 is preferably circular when viewed from above. The average diameter of the dimples may be set to at least about 2.8 mm, more preferably at least about 3.5 mm, and still more preferably at least about 3.8 mm. The upper limit thereof may preferably be about 5.0 mm, more preferably about 4.6 mm, and still more preferably about 4.3 mm. The average depth of the dimples 32, from the standpoint of achieving an adequate trajectory, may be preferably at least about 0.120 mm, more preferably at least about 0.130 mm, and still more preferably at least about 0.140 mm. The upper limit of the average depth may be preferably about 0.185 mm, more preferably about 0.180 mm, and still more preferably about 0.174 mm.

The average diameter means the average of the diameters of all the dimples, and the average depth means the average of the depths of all the dimples. In many cases, the golf ball is painted, and the diameter and the depth of the dimple are measured with the paint coating applied. The measurement of the diameter of the dimples is performed by measuring the width bridged between the points connecting with the land part which is the surface of the glove ball on which the dimples are not formed and the graved surface. The measurement of the depth of the dimple is performed, when the points connecting the above dimple and the land part are linked each other on an imaginary plane circle, by measuring vertically the distance between the center thereof to the base surface of the dimple.

The dimple-occupied surface ratio, which is specifically the ratio (SR value) of the total area of the dimples which is defined by the plane edges surrounding the borders of the dimples to the spherical surface area of the ball assumed to have no dimples, from the standpoint of fully exhibiting aerodynamic characteristics, is preferably at least about 60%, more preferably at least about 65%, and still more preferably at least about 68%. Although the upper limit of the dimple-occupied surface ratio is not limited, it is preferably about 90%, more preferably about 85%, and still more preferably about 80%. Also, the spatial volume of each dimple below the flat surface surrounded by the border thereof, when divided by the volume of a cylinder whose base is the flat surface and whose height is the maximum depth of the dimple from the base equals the value V₀, and from the standpoint of optimizing the trajectory of the ball, is preferably at least about 0.35. Although the upper limit of the V₀ is not particularly limited, it may preferably be about 0.80. Also, the VR value, which is the proportion of the total volume of the dimples, which are formed below the flat surface surrounded by the border of the dimples to the spherical volume of the ball assumed to have no dimples, is preferably at least about 0.6%, more preferably at least about 0.7%, and still more preferably at least about 0.8%. The upper limit of the VR value is preferably 1.0%, and more preferably 0.9%.

EXAMPLES

Golf balls constituted as shown in Table 1 were made, and an experiment was performed to measure the moment of inertia, the distance, and the spin decay rate of the golf balls. The test results are shown in Table 1. The details of mixtures A to E (wt %) for the materials of the solid core are shown in Table 2. The details of mixtures F to H (wt %) of materials for the intermediate layer and the cover are shown in Table 3. In Example 1, as shown in FIG. 3, the center of the golf ball and the center of the spherical specific gravity-enhancing member were disposed identically. In Example 3, six disc-like specific gravity-enhancing members (the total weight of 6 members being 1.5 g) having a thickness of 0.3 mm and a diameter of 12 mm were disposed at the centers of faces of the regular hexahedrons and on the surface of the solid core as well.

TABLE 1 Compar- Exam- Exam- Exam- Exam- Exam- ative ple 1 ple 2 ple 3 ple 4 ple 5 Example Solid Center Outer diameter (mm) 27.0 15.3 37.0 25.1 37.2 core core Specific gravity 1.6 3.2 1.2 2.8 1.0 Weight (g) 16.5 6.0 31.8 24.1 28.0 Mixture A B C L D Core Outer diameter (mm) 20.0 layer Specific gravity 1.4 Weight (g) 4.4 Mixture E Specific Outer diameter (mm) 12 gravity- Specific gravity 7.2 7.2 enhancing Weight (g) 6.5 1.5 member Mixture or material K J Intermediate Outer diameter (mm) 40.7 40.7 40.7 37.0 40.7 layer 1 Specific gravity 0.94 0.94 0.94 0.43 1.30 Weight (g) 29.1 23.4 31.3 7.8 10.7 Mixture F F F N H Intermediate Outer diameter (mm) 40.6 layer 2 Specific gravity 0.94 Weight (g) 7.9 Mixture G Cover Outer diameter (mm) 42.7 42.7 42.7 42.7 42.7 42.7 Specific gravity 0.94 0.97 1.16 0.94 0.94 1.16 Weight (g) 5.3 5.4 6.5 13.4 5.5 6.5 Mixture F G I F F I Golf ball Weight (g) 45.3 45.3 45.3 45.2 45.3 45.2 Moment of inertia 72 76 75 80 67 84 Dimples Overall number 336 336 336 336 336 336 Average diameter 4 4 4 4 4 4 (mm) Average depth (mm) 0.161 0.161 0.161 0.161 0.161 0.161 Number of types 9 9 9 9 9 9 Distance 2800 rpm, 8deg 0.9 0.6 0.6 0.3 1.1 0 (m) 2800 rpm, 11deg 1.4 0.9 0.9 0.4 1.6 0 3500 rpm, 11deg 1.2 0.8 0.8 0.3 1.9 0 Spin decay rate (%) 3.5 3.4 3.4 3.3 3.6 3.0

TABLE 2 A B C D E L BR730 100 100 100 100 100 100 Zinc oxide 104.3 5.0 28.5 1.9 64.6 450 Tungsten — 330 — — — — Zinc diacrylate 25 25 25 25 25 25 Zinc stearate G 5 5 5 5 5 5 Perhexa C-40 1.2 1.2 1.2 1.2 1.2 1.2 Organosulfur compound 0.4 0.4 0.4 0.4 0.4 0.4 Nocrac NS-6 0.1 0.1 0.1 0.1 0.1 0.1

BR730 is the trade name of 1,4-cis-polybutadiene available from JSR Corporation, which was used as the base rubber.

Zinc oxide is available from Sakai Chemical Industry Co., Ltd.

Tungsten is available from Nippon Tungsten Co., Ltd.

Zinc diacrylate is available from Nippon Shokubai Co., Ltd.

Zinc Stearate G is the trade name of zinc stearate available from NOF Corporation, which was used as a vulcanization accelerator.

Perhexa C-40 is the trade name of 1,1-bis(tert-butylperoxy)cyclohexane (40% dilution) is available from NOF Corporation, which was used as an initiator.

The organosulfur compound is a pentachlorothiophenol zinc salt.

Nocrac NS-6 is the trade name of 2-2′-methylenebis(4-methyl-6-t-butylphenol) available from Ouchi Shinko Chemical Industry Co., Ltd., which was used as an antioxidant.

TABLE 3 F G H I N Himilan 1605 66 — — — Dynaron 6100P 32 — — — Polytail H 2 — — — Behenic acid 18 — — — Calcium hydroxide 2.3 — — — Himilan 1557 — 75 75 — 75 Himilan 1855 — 25 25 — 25 Magnesium stearate — 1.8 1.8 — 1.8 Titanium white — 3.8 3.8 — 3.8 Barium sulfate 300 — — 48.1 — Pandex T8295 — — — 22 Pandex T8290 — — — 65 Hytrel 4001 — — — 13 Polyisocyanate — — — 8 compound Foaming agent 3

Himilan 1605 is the trade name of an ionomer resin available from DuPont-Mitsui Polychemicals Co., Ltd.

Dynaron 6100P is the trade name of a hydrogenated thermoplastic elastomer available from JSR Corporation.

Polytail H is the trade name of polyolefin polyol available from Mitsubishi Chemical Corporation.

Behenic acid is available from NOF Corporation.

Calcium hydroxide is available from Shiraishi Kogyo Kaisha Ltd.

Himilan 1557 is the trade name of an ionomer resin available from DuPont-Mitsui Polychemicals Co., Ltd.

Himilan 1855 is the trade name of an ionomer resin available from DuPont-Mitsui Polychemicals Co., Ltd.

Barium sulfate 300 is the trade name of barium sulfate available from Sakai Chemical Industry Co., Ltd.

Pandex T8295 is the trade name of a polyurethane thermoplastic elastomer available from DIC Bayer Polymer.

Pandex T8290 is the trade name of a polyurethane thermoplastic elastomer available from DIC Bayer Polymer.

Hytrel 4001 is the trade name of a thermoplastic polyether ester elastomer available from DuPont-Toray Co., Ltd.

The polyisocyanate compound is 4,4′-diphenylmethane diisocyanate.

The foaming agent is the ADCA master batch available from Otsuka Chemical Co., Ltd.

Material J for the specific gravity-enhancing member is a lead sheet with adhesive backing which is available from Osaka Kako Co., Ltd. Mixture K for the specific gravity-enhancing member includes tungsten of 300 parts by weight, which is available from Nippon Tungsten Co., Ltd. (powder-type tungsten), BR730 of 40 parts by weight, which is 1,4-cis-polybutadiene available from JSR Corporation, LIR410 of 60 parts by weight, which is a liquid-type isoprene rubber available from Kuraray Co., Ltd., and Sulfur Z of 2 parts by weight, which is sulfur available from Tsurumi Chemical Industry Co., Ltd.

The moment of inertia of the golf ball was measured using MOI-005, which is an instrument for calculating the moment of inertia and is available from Inertia Dynamics Inc. This measuring instrument measures the moment of inertia of a golf ball by means of the difference between a period of oscillation of the golf ball when the golf ball is mounted in a fixture of the measuring instrument and a period of oscillation of the golf ball when the golf ball is not mounted therein.

The distance of the golf ball was tested using the UBL which is a launcher available from Automated Design Corporation. The test was performed under the conditions of an initial velocity of the ball of 59 m/s, the initial spin rate of 2800 rpm or 3500 rpm, and the launch angle of 8 degrees or 11 degrees. The results of the distance test in Table 1 show improved distances (unit: m) of Examples compared with the distance of Comparative Example. The UBL is an apparatus having two pairs of drums, top and bottom, the top and bottom pairs of drums having belts wound therearound, a ball being inserted therebetween, and launched under desired conditions.

The spin rate of the golf ball during flight was measured using TrackMan which is the trade name of a golf ball trajectory tracking system available from Trackman A/S. The spin loss of the ball per one second was determined by the slope of an approximate straight line, which was calculated from the measured data mentioned above by using the least-squares method. The spin decay rate of the golf ball was obtained by dividing the spin loss per one second by the initial spin rate of the golf ball.

As shown in Table 1, Example 1 resulted in extending the distance longer by 0.9 m to 1.4 m compared with Comparative Example, the golf ball of Example 1 including a spherical specific gravity-enhancing member disposed at the center of the solid core and including a core layer having a high specific gravity of 1.4 disposed around the solid core, and thus having a high spin decay rate of 3.5%, whereas the golf ball of Comparative Example had a low specific gravity of the center core of 1.0 and had a low spin decay rate of 3.0%. Example 2 resulted in extending the distance longer by 0.6 m to 0.9 m compared with Comparative Example, the golf ball of Example 2 including a center core having a high specific gravity of 1.6, and thus having a high spin decay rate of 3.4. Example 3 resulted in extending the distance longer by 0.6 m to 0.9 m compared with Comparative Example, the golf ball of Example 3 including plural specific gravity-enhancing members and a center core having a high specific gravity of 3.2, and thus having a spin decay rate of 3.4%. Example 4 resulted in extending the distance longer by 0.3 m to 0.4 m compared with Comparative Example, the golf ball of Example 4 including a center core having a high specific gravity of 1.2 and having a high spin decay rate of 3.3%. Example 5 resulted in extending the distance longer by 1.1 m to 1.9 m compared with Comparative Example, the golf ball of Example 5 including a center core having a high specific gravity of 2.8 and a center-side intermediate layer having a low specific gravity of 0.43, and thus having a high spin decay rate of 3.6%.

LEGEND FOR REFERENCE NUMERALS

1: Golf ball

10: Solid core

12: Center core

14 and 16: Core layer

20: Intermediate layer

30: Cover

32: Dimple

40: Specific gravity-enhancing member

42: Specific gravity-enhancing member

44: Specific gravity-enhancing member 

1. A multiple-piece golf ball comprising: a solid core having a specific gravity from about 1.2 to about 14; and a cover disposed on the outside of the solid core, wherein a spin decay rate of the golf ball during flight is from about 3.1% to about 20% of an initial spin rate of the golf ball from about 2800 to about 3500 rpm; and wherein a moment of inertia of the golf ball with respect to an x-axis Ix, a moment of inertia of the golf ball with respect to a y-axis Iy, and a moment of inertia of the golf ball with respect to a z-axis Iz are substantially the same, the x-axis, y-axis, and z-axis intersecting orthogonally at the center of the golf ball.
 2. The golf ball according to claim 1, wherein the solid core comprises at least one specific gravity-enhancing member.
 3. The golf ball according to claim 2, wherein the solid core comprises one specific gravity-enhancing member.
 4. The golf ball according to claim 2, wherein the solid core comprises plural specific gravity-enhancing members.
 5. The golf ball according to claim 3, wherein the specific gravity-enhancing member has a transmission loss of at least about 10 dB in the region from 1000 to 4000 Hz.
 6. The golf ball according to claim 4, wherein the specific gravity-enhancing member has a transmission loss of at least about 10 dB in the region from 1000 to 4000 Hz. 